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So a 2,000-square-foot crawl space would need a 40 cfm exhaust fan. It's pulling just a little bit of air from the crawl space. The problem is that you don't really know where the makeup air for the crawl space is coming from. Yes, the code specifies a pathway for air to move into the crawl space from the house, but air doesn't always follow those arrows in the diagrams.
The exposed dirt floor emits moisture into the air, this is compounded by the high summer humidity that is delivered via the crawl space foundation vents and gaps in the rim joist and sill plate. Research has shown that water vapor in the air is exchanged simultaneously between the outside air and the crawl space. In other words the moisture in the air outside your crawl space will equal the moisture in the air inside your crawl space at all times regardless of whether the metal vents are open or closed.